This invention relates to processes in which suspended solids in a mineral suspension, usually an aqueous mineral suspension, are separated by sedimentation. In particular it relates to such processes in which the sedimentation is conducted in such a manner as to prevent a layer of solids such as a thick crust forming on the upper surface of the suspension, and the apparatus containing it, during the sedimentation process.
Sedimentation processes for suspended solids in mineral suspensions are well known and conventionally are promoted by the addition of a flocculating agent. Sometimes a significant amount of fines or other material remains or accumulates on the surface of the suspension, often as a thick crust, during the flocculation and sedimentation. This occurs particularly when the suspension has been produced at least partially from a flotation process.
A particular problem arises when the suspension in one which is generated in the recovery of phosphate from a phosphate clay mineral. During this process phosphate values are separated from phosphate clay ores in various initial separation processes to separate the clay fraction from the phosphate and quartz sand fractions. The enriched phosphate fraction is subjected to further processing which results in the generation of additional clay waste, and is then subjected to one or more stages of preferential flotation. In this flotation stage the reject fraction containing primarily quartz sand solids, is often called the "tailings". The accept fraction contains phosphate values. During the flotation processes, the fraction containing phosphate values is subjected to various washing stages. During these washing stages flotation reagents are washed or stripped from the phosphate rich fraction and the residual reagents and associated wash water are combined with the collective clay waste suspensions from different stages of the phosphate recovery process. The combined suspension is then subjected to sedimentation for disposal of the clay solids and for reuse of the aqueous phase. However, clay solids and spent flotation reagents tend to form a thick crust (which can be up to 12 inches thick) on the surface of the suspension during the sedimentation process. As a result clay solids tend to be carried into the overflow water rather than remaining in the underflow. This means that the overflow water cannot be recycled directly into the plant without the risk of causing poor performance in one or more of the processes described above unless subjected to further treatment.
This surface solids formation has been a serious problem in various mineral sedimentation applications, in particular phosphate clay sedimentation, for many years and has not been solved.
Crust formation in other systems has been addressed in various ways. Anionic surfactants have been suggested for reducing surface crust formation in aqueous sodium hydrosulphite dye reducing compositions (in U.S. Pat. No. 4,285,695). Solutions for phosphating metal based sludges have been provided with alkali metal or ammonium lignosulphate additives to prevent crust formation on apparatus used for the phosphating process (in U.S. Pat. No. 4,147,567). However, neither of these involves separation of mineral solids from a suspension by sedimentation using an anionic flocculating agent.
In other processes which do not appear to present a problem with surface solids formation combinations of flocculant and surfactant have been used. For instance JP63/291700, JP61/204098, JP61/078499 and JP50/01967 disclose treatment of organic sludge from waste water treatment using flocculant and a surfactant. The inclusion of the surfactant in JP63/291700 is said to improve the handling properties of the flocculant. JP03/270780 describes a process of flotation of sludge in which surfactant and flocculant are added during the flotation process. JP62/298492 describes the addition of an inorganic salt (i.e., an inorganic coagulant) and a surfactant to a waste water dyeing solution. pH is adjusted to induce precipitation of materials from the solution and flocculant is added. The precipitate obtained is then separated from the solution.
However, none of these publications addresses the problem of surface solids formation in sedimentation processes for mineral suspensions which are treated with a flocculant to improve sedimentation.